Semiconductor lasers are in practical use in many fields. Japanese Unexamined Patent Publication No. 2005-055881 discloses a laser exposure apparatus that modulates light emitted from semiconductor lasers with spatial light modulating elements, and exposes photosensitive materials with the modulated light.
In addition, GaN type semiconductor lasers that emit laser beams having wavelengths in the vicinity of 400 nm are known, as disclosed, for example, in Japanese Unexamined Patent Publication No. 2004-096062. Japanese Unexamined Patent Publication No. 2005-055881 discloses that the exposure apparatus employs this type of semiconductor laser as an exposure light source.
In applications of semiconductor lasers, such as when they are employed in the aforementioned exposure apparatus, it is desired to drive the semiconductor lasers such that their light output is constant. Known methods for driving semiconductor lasers in this manner include an ACC (Automatic Current Control) drive method, and an APC (Automatic Power Control) drive method, as disclosed in Japanese Unexamined Patent Publication No. 8 (1996)-274395. Note that Japanese Unexamined Patent Publication No. 2001-267669 discloses a driving method for semiconductor lasers, in which the semiconductor lasers are driven by the ACC method immediately following initiation of drive of the semiconductor lasers such that the drive current value is gradually increased, then driven by the APC method thereafter.
The drive current/light output properties of semiconductor lasers change due to self heating and the like. Therefore, it is recognized that the ACC drive method, in which drive current is controlled to be constant, has a drawback that the light output changes after the laser is turned on. This defect occurs particularly conspicuously in high output semiconductor lasers. Similarly, this defect occurs conspicuously in laser apparatuses, in which pluralities of semiconductor lasers are mounted. Further, blue-violet GaN type semiconductor lasers have poorer luminous efficiency and generate a greater amount of heat than red lasers. Therefore, the light output changes more conspicuously in blue-violet GaN type semiconductor lasers.
In view of these circumstances, the APC drive method is generally employed to obtain stable light output. In the APC drive method, the drive current is controlled by: causing a portion of a laser beam emitted by a semiconductor laser to enter a monitoring photodetector; and creating a feedback loop such that a monitor current, which is generated proportionate to the light output of the semiconductor laser, becomes constant.
In the APC drive method, however, a portion of the emitted light is utilized as an input to the feedback loop, causing a drawback that the amount of light to be utilized for the intended purpose is reduced. In addition, additional costs become necessary to provide a light amount feedback loop circuit.
Further, in the APC method, there is a problem that light output will not stabilize under specific conditions. That is, semiconductor lasers are generally constituted by semiconductor laser chips housed within a Can type package. Monitoring photodetectors for detecting light emitted toward the rear of the semiconductor chips are also housed within the package. The APC method is normally realized by employing the monitoring photodetectors, to obtain stabilized light output. However, there are cases in which light output does not stabilize even if the APC drive method is adopted, particularly when the semiconductor lasers are high output lasers, such as GaN type semiconductor lasers.
This is because the quantum efficiency of the photodetectors such as photodiodes, which are placed in the vicinity of the semiconductor laser chips, change due to the heat generated by the semiconductor laser chips. Accordingly, the light input amount/output properties of the photodetectors change. In this case, it is difficult to stabilize light output even if the drive method that employs both the ACC drive method and the APC drive method disclosed in Japanese Unexamined Patent Publication No. 2001-267669 is employed.
Meanwhile, in laser exposure apparatuses such as that described above, the light output of the semiconductor lasers is a factor that determines the takt time of exposure processes. Therefore, obtainment of stable high output laser beams at low cost is desired. However, in the case that the ACC drive method is employed to obtain stable light output, the laser exposure apparatuses must stand by until the temperature of the semiconductor lasers stabilizes after they are turned ON. This generates loss in production time, thereby increasing the takt time of the laser exposure apparatuses. The increase in the takt time deteriorates the productivity of exposure processes.
Constantly keeping the semiconductor lasers in an ON state may be considered as a method to eliminate the time loss caused by the aforementioned standby state. However, the life of lasers is determined by the amount of time that they emit light. Therefore, the effective life of the semiconductor lasers is reduced by the amount of time that they are in the ON state and are not being utilized to perform exposure processes. In the case that the percentage of time that the lasers are utilized for exposure within the total time that the lasers in the ON state is 50%, for example, the life of the semiconductor lasers is reduced by approximately ½.
The present invention has been developed in view of the foregoing circumstances. It is an object of the present invention to provide a method and apparatus for driving semiconductor lasers which enables obtainment of stable high output laser beams simply, at low cost, and without a long startup time.
It is another object of the present invention to provide a method and apparatus for deriving correction patterns, which are employed in the method and apparatus for driving semiconductor lasers, for correcting the set value for automatic power control and/or the output of photodetectors.
It is still another object of the present invention to shorten the takt time of exposure apparatuses that expose photosensitive materials with light, which is emitted from semiconductor lasers and modulated by spatial light modulating elements.